The present invention relates generally to methods for manufacturing contact lenses, and more particularly to methods for manufacturing finished aspheric single vision contact lenses, or finished spherical or aspheric multifocal contact lenses.
As used herein, the term "multifocal" is used to generally refer to bifocal contact lenses, trifocal contact lenses, progressive contact lenses, and so forth.
While contact lenses are worn by over 10% of all antimetropes in the U.S., multifocal contact lenses have enjoyed only a mixed success. No multifocal contact lens has been successfully accepted by more than 70% of the patients fitted with a particular design.
Currently available multifocal contact lenses are designed based on the assumption of an exact fit. An exact fit as defined herein means that the contact lens will be centered with respect to the center of the patient's pupil. In practice, however, since contact lenses are made with a limited number of concave curvatures, the fit is almost never exact for an individual patient. Instead, the contact lens positions itself on the cornea at a position determined by the difference between the curvature of the cornea and the curvature of the contact lens. The relationship between the lens and the cornea is also affected somewhat by other factors such as lid tension, tear rate and so forth.
So long as the lens is of a single vision type, this accentricity between the pupil and contact lens is too small to cause any significant change in the refractive correction provided by the lens. However, patients wearing other types of lenses such as bifocal contact lenses can suffer a significant loss of visual acuity or contrast due to this accentricity. This loss of visual performance of bifocal contact lenses can occur in every bifocal design, although the deleterious effects of decentration on visual acuity as a function of contrast may be mitigated to some extent by centered diffractive bifocal designs. Therefore, lack of perfect fit, which leads to decentration of the lens, is the leading cause of patient maladaptation to bifocal contact lenses, whether made of soft hydrophilic materials or of hard gas permeable materials.
One solution to the above problem as recognized by the inventors is to provide the patient with a lens having a perfect fit. A perfect fit, however, requires a perfect match between the corneal curvature and the concave curvature of the contact lens. The corneal curvature of each individual is unique, and often has zones of pronounced asphericity. Therefore, it is not practical to stock lenses matching all possible corneal topographies. Moreover, there are also problems associated with customizing the concave surface of each contact lens based on the corneal curvature, because altering the concave curvature would change the optical characteristics, i.e., the spherical power or astigmatic correction, provided by the lens.
The reason why the add power zone should be within the pupillary aperture is that for a multifocal lens to function properly, the retina should receive all the images at the same time. For distant objects, the image formed by the base power zone is focused, while the image formed by the add power zone is defocused. For near objects, the image formed by the base power zone is defocused, while the image formed by the add power zone is focused. Given one focused and one or more defocused images, the image processing apparatus at the retina and the visual cortex rejects the unfocused images and processes the focused image.
Persons with normal accommodation not requiring any refractive correction also receive multiple images simultaneously at their retina, and possess the ability to ignore the defocused image of far objects when looking at near objects, and vice versa. This analogy to a normal eye indicates that for a multifocal contact lens to work properly, the add power zone should be within the pupillary aperture. Since image strength at the retina is proportional to the area of the corresponding refractive zone (i.e., add or base power) subtended at the pupil, the optimum area of the add power zone can be computed with respect to the pupil size. It is known that pupil size varies from person to person and also depends on the level of ambient illumination and physicochemical status of the individual. For example, the pupil size of a thirty year old can vary from 2.2 mm in direct sunlight to 5.7 mm outdoors at night. Data on pupil size distributions by age and illumination level are available in the literature. The assumption may also be made that the contact lens wearer will generally be outdoors when experiencing extreme levels of illumination, where distance vision will be needed the most, whereas ambient illumination is at an intermediate level indoors, where near and intermediate vision is required most often. Based on these considerations, it is possible to develop a model which predicts the optimum sizes of add power zones for near vision, base power zones for distance vision and aspheric zones for intermediate vision, if needed. Such a model is disclosed in U.S. Pat. No. 5,112,351.
When positioning a multifocal segment on the patients eye, it is typically sufficient to locate the multifocal segment within the pupillary aperture based on the center of the patients pupil. However, it is sometimes desirable to locate the multifocal segment based on the line of sight, or based on some other optical feature of interest. For example, although the line of sight of the eye tends to correlate to the center of the patients pupil, the line of sight can nonetheless deviate from the pupil center in some cases. In cases where the line of sight deviates from the center of pupil, it is useful to position the multifocal portion of the contact lens with respect to the line of sight, rather than the center of the pupil.
In other circumstances, it is also useful to position the toric surface of a contact lens with respect to the pupillary center or the line of sight of an astigmatic user. For example, for best adaptation, it is preferable to orient the axis of the astigmatic correction of toric contact lens at right angles to the corneal asphericity at the rest or equilibrium position of the lens on the patient's eye. Moreover, it is desirable that these axes cross at the center of the pupil or at the line of sight, for example.
Thus, in view of the above, there is a need for a contact lens where the add power zone or toric zone is precisely positioned with respect to an optical feature associated with the patient's eye, and for a process for making the same.